Freshwater golden clam

What does it look like?

The term “freshwater gold clam” is no longer preferred by some authorities, as the goldcolouring is not always obvious. The name invasive freshwater clam is now commonly used, as there are no native New Zealand freshwater shellfish that resemble this species.

Adult invasive freshwater clams are two to three centimetres long and are typically dirty white, yellow or tan in colour, with a distinctive ribbed shell. They are found in freshwater and brackish water, either sitting on or shallowly buried in sandy or muddy substrates.

The larvae are extremely small (about 200 microns - 0.2 mm) and are effectively invisible to the naked eye. This makes them easy to spread accidentally on boats and recreational equipment, including white baiting gear.

High‑risk locations include waterways used by boats that may have been in Lake Karapiro. Whitebaiters may also spread larvae by moving nets between waterways.

Why is it a problem?

Invasive freshwater clams reproduce rapidly and in very large numbers. As hermaphrodites, a single clam can self fertilise and produce thousands of juveniles, allowing populations to grow quickly. They can outcompete native species for food and habitat, posing a potential threat to native freshwater mussels (kākahi).

These clams are filter feeders and can compete directly with native species for food. By altering nutrient cycling and sediment composition, they can change ecosystem processes and reduce water quality. In addition to ecological impacts, invasive freshwater clams can cause significant economic damage by clogging water intake pipes and machinery, increasing maintenance costs for power generation and water treatment infrastructure.

It’s not yet known how invasive freshwater clams will behave under New Zealand conditions. However, overseas experience shows they tolerate a wide range of environmental conditions, spread rapidly, and are extremely difficult to control. Eradication has not been achieved anywhere they have established.

Whitebaiting gear - nets, screens and spotter boards - is particularly difficult to decontaminate. The biosecurity risk is not whitebaiting itself, but the movement of fishing gear between waterways, which can spread microscopic larvae.

While the potential impacts on the whitebait fishery are currently unknown, preventing the spread of invasive freshwater clams is critical to protect native fish, freshwater ecosystems and cultural values, even where impacts are not yet fully understood.

Control methods

The best way to protect our waterways is to Check, Clean, dry your gear and watercraft every time you leave the water.

Check
Remove any plant matter, mud, or visible clams from your gear.
Leave unwanted material at the river or lake bank or put it in the rubbish.
Drain all river or lake water from your boat and gear before leaving.

Clean
Blast your gear and craft with tap water onto grass (never into stormwater drains). For absorbent materials, use one of these treatments (always follow manufacturer’s instructions):
Hot water soak: 50–55°C for at least 5 minutes
Bleach soak: 5% solution (250ml bleach in 4.75L water) for 1 hour
Freezing: until solid (overnight)
Salt soak: 100ppt solution (500g salt in 5L warm water) for 4 hours

Dry
Gear: Dry to the touch inside and out, then leave for at least 48 hours (2 days) before reuse.
Watercraft: Dry pooled areas inside with a towel, then leave for at least 48 hours (2 days). The hull will dry naturally when towed.

Currently, gold clams are only confirmed to be in the Waikato River. Since the initial discovery, national surveillance has been underway to determine the extent of the population.
Biosecurity NZ (BNZ) is the lead agency in the early stages of an incursion response to the discovery of gold clams. DOC and stakeholders – including river iwi, Waikato Regional Council, Waikato River Authority, Toitū Te Whenua Land Information New Zealand, Mercury Energy, and others – are supporting BNZ as part of the response.
Early detection allows a quick response and provides an opportunity to eliminate Corbicula from other areas where newly detected populations are not well established. For more information about the freshwater invasive clam or to report a sighting contact Biosecurity New Zealand 0800 80 99 66.

More information

The freshwater golden clam was discovered in the Waikato River in May 2023. It is unknown how long the organism has been in NZ, the extent of its spread, nor how it was introduced to New Zealand.

Gold clams breed rapidly and can occur in dense populations in suitable habitats. The clams have been found in wading depth water, in moderately coarse sediments of the Waikato River. They can potentially exist in brackish habitats as well as fresh water. Based on current knowledge, the clams are most likely to spread between waterbodies by people boating and recreating.

In mid-August 2023, the gold clam was given the legal status of an Unwanted Organism under the Biosecurity Act. As an Unwanted Organism, gold clam (and water potentially containing it), must not be knowingly moved or spread.

A national surveillance programme including risk sites (which includes locations with significant movements of people and water equipment from the Waikato River) is managed under the long-term programme. It includes ongoing sampling of waterways to help identify if there has been spread beyond known locations.

Testing of these water bodies in both the North and South Islands continues in partnership with regional councils and iwi.

Related links

• https://www.mpi.govt.nz/outdoor-activities/boating-and-water-activities-preventing-the-spread-of-pests-weeds-and-diseases/check-clean-dry-for-the-north-island
• https://www.mpi.govt.nz/biosecurity/exotic-pests-and-diseases-in-new-zealand/long-term-biosecurity-management-programmes/invasive-freshwater-clam-corbicula
• https://www.doc.govt.nz/nature/pests-and-threats/freshwater-pests/gold-clams/
• https://niwa.co.nz/freshwater/stopping-gold-clam-its-now-or-never